Security researchers said Monday they have discovered a critical flaw in the way certain email programs handle a popular encryption technology that safeguards emails from prying eyes.
The flaw, known as EFAIL, affects applications such as Mozilla Thunderbird, Apple Mail and some versions of Outlook, said the team of European researchers. EFAIL targets the encryption standard known as PGP, or Pretty Good Privacy, and S/MIME, a similar protocol commonly used by enterprises. (A full list of affected email programmess is available from the researchers' report).
Whistleblowers, political activists and others who depend on encrypted email could all be compromised by the bug, the researchers said in a blog post. The Electronic Frontier Foundation, a separate technology advocacy group that previewed the researchers' findings on Sunday, said users of the affected email programs should disable any third-party software they have installed that allow the email apps to use PGP or S/MIME. (EFF has provided step-by-step instructions for each type of mail client.)
"Until the flaws described in the paper are more widely understood and fixed," EFF said, "users should arrange for the use of alternative end-to-end secure channels, such as Signal, and temporarily stop sending and especially reading PGP-encrypted email."
The flaw works when an attacker already has access to a victim's encrypted emails. The vulnerability allows hackers to read an encrypted email by making changes to its HTML, which essentially tricks the affected email applications into decrypting the rest of the message.
Apple and Microsoft didn't immediately respond to a request for comment. Mozilla referred questions to the Thunderbird Council, the third-party open-source software group that maintains the Thunderbird email app. Ryan Sipes, a Thunderbird community manager, said in a statement that a patch is being developed and will be distributed as an update by the end of the week.
Some security experts said that because EFAIL seems to affect specific email applications, it is overkill to say that there is a flaw in the actual underlying encryption protocols.
Werner Koch, the principal author of the cryptographic software GNU Privacy Guard, called EFF's warnings about the vulnerability "pretty overblown." In a post-Monday, he said his team wasn't contacted about the flaw and the attack could be mitigated by avoiding HTML emails or using authenticated encryption, which adds a layer of protection to confirm the message hasn't been changed. Still, some developers of PGP software for email apps aren't taking any chances.
GPGTools tweeted "'Efail': as a temporary workaround against 'efail' ... , disable 'Load remote content in messages' in Mail → Preferences → Viewing.
"GPG Suite 2018.2 which mitigates against this attack is coming very soon."
Rather than deal with email encryption issues at all, others said, just switch to an encrypted messaging app that doesn't require any third-party plugins.
Barton Gellman tweeted "The best advice TBH is just to stop using GPG / PGP (for most purposes) and start using Signal. Safer, easier, free, works on your phone at least as well as on a computer. Messages, attachments, audio or video calls. Just get it."